The removal of dust is accomplished by passing the air through a filter of material permeable to flow of gas but not to passage of particulate matter, which collects on the filter thus gradually filling its pores and increasing the restriction of the cleaner, that is, the pressure drop across the filter and the load on the air-moving fan or blower. A successful air cleaner must accordingly have a large enough area of filter medium to reduce the initial restriction to an acceptable level, and must be either cleaned or replaced at sufficiently frequent intervals to prevent dirt buildup to a point where the restriction is adversely affective.
Means have been devised for cleaning filters, even without interrupting system operation, by mechanical shaking or by reverse air jet pulsing. The latter cleaning procedure is successful, when the filter medium is pleated paper, in releasing the particulate matter from the medium, but the resumption of normal airflow through the filter at the end of each pulse in large measure draws the particles back against the filter medium, thus greatly reducing the cleaning efficiency. This is particularly noticeable in installations which because of the large volume of air to be treated require large areas of filter medium.